1. Field of the Invention
The present invention relates to an inertial sensor which mainly detects an acceleration, and a combined sensor therewith.
2. Description of the Related Art
In recent years, a system has become popular which conducts extremely precise navigation and controls the safety of a vehicle by detecting the vehicle's acceleration. In such circumstances, various sensors have been presented which are capable of detecting an acceleration or an angular velocity as an inertial quantity of a vehicle. There is also a future sign that several sensors will combine together to produce a low-priced sensor with a high-level function.
In some of the conventional acceleration sensors of the electrostatic capacity type, changes in two electrostatic capacities (i.e., the capacities of two capacitors) are used which are generated between opposite electrodes placed near a cantilever that is displaced according to the applied acceleration.
As such a sensor, for example, there is known an acceleration detector which is described in Japanese Unexamined Patent Publication (kohyo) No. 4-504003 specification. In this acceleration detector, as shown in FIG. 10, two capacitors 101, 102 are bridge-connected in series. From an oscillator 103, an AC-bias signal voltage is applied to the bridge which is made up of these capacitors 101, 102. Thus, an acceleration is applied, and thereby, a cantilever (not shown) is displaced. Therefore, when this cantilever is displaced, a divided-voltage electric potential is generated at a connection point 104 of the bridge, and then, it is detected by an amplifier 105. Next, it is rectified by a rectifier 106, and finally, it is amplified by an LPF 107. Thereby, a signal can be obtained which corresponds to the applied acceleration.
As another such sensor, a uni-axial angular-velocity/acceleration sensor is known which is described in Japanese Unexamined Patent Publication No. 7-128355 specification. In this sensor, as shown in FIG. 11, resistors 110, 111 are connected in series to two capacitors 108, 109, respectively, so that it has the configuration of a CR bridge. To this CR bridge, an AC-bias signal voltage is applied from an oscillator 117. Thus, an acceleration is applied, and thereby, a cantilever (not shown) is displaced. Therefore, when this cantilever is displaced, the balance of an impedance is changed at connection points 112, 113 of the CR bridge. Then, the electric potential at each connection point 112, 113 is detected by a differential amplifier 114. Next, it is rectified by a rectifier 115, and finally, it is amplified by an LPF 116. Thereby, a signal can be obtained which corresponds to the applied acceleration.
In the above described conventional acceleration sensor of the electrostatic capacity type, a signal which corresponds to the applied acceleration can be obtained. However, whether or not there is an abnormality in the sensor itself cannot be actively diagnosed.